Which Syn GL-5 for MT Where Front Diff Shares Oil?

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LoneRanger

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2009 Subaru, manual transmission. Front differential is in the trans and shares the manual trans fluid, so does center differential (AWD). Want to change out the factory OE manual trans oil for synthetic, and also change rear diff oil to synthetic. Front and rear diffs are open type, not LSD.

Subaru says use GL-5 gear oil in both the rear diff (capacity .8 qts) and the manual transmission (capacity 3.7 qts) 75W90 recommended for both.

Was considering Mobil 1 Synthetic Gear Lubricant LS 75W-90 for the rear diff even though it has the limited slip friction modifiers it should be okay for the hypoid geared open rear diff.

But for the manual trans I'm not sure what to use. I know you're not supposed to use any gear oil containing the limited slip friction modifiers in a manual trans, so I need to find a 75W90 GL-5 syn gear oil that will keep the manual trans, front diff, and center diff all happy and long lived.
 
Red Line and Amsoil make good 75w-90s without limited slip additives.

RLI makes a 75w-90, too. My experience with their gear oils is limited, though (no pun intended).

I wouldn't use the Mobil 1 LS in an open diff. There are plenty of good 75w-90s without FMs.

Can you use the same fluid in the front transaxle and rear diff? That'd be convenient.
 
I'm in the same boat with my '08 STI. It seems that a lot of people like the Motul 300V 75W-90 all around, some the valvoline dino and others think Go Extra S would be just fine. It seems that any GL-5 is fine, but I'm still not convinced that a GL-5 is the best choice for the shared tranny/front diff. A lot of GL-5 gear oils seem to be touting their compatibility with yellow metals such as synchros, but I haven't read enough to determine which is best just yet. : /
 
300V is engine oil. Did you mean Gear 300?

Great fluid, by the way. Should have included it in my list.
thumbsup2.gif
 
Originally Posted By: d00df00d


Can you use the same fluid in the front transaxle and rear diff? That'd be convenient.


According to the owner's manual as well as the 2009 Forester Factory Service Manual, yes. Both of them specify the same gear oil type for rear diff and for the manual trans: API GL-5, @ 75W90 'recommended.' They have the usual ambient temperature chart with 75W90 covering the broadest range of course.
 
For the rearend, any GL5 full synthetic.

For the manual tranny, find one without LSD additive like Redline 75w90NS or Motul Gear300.
Other options are Ford's XT-75W90-QGT, Silkolene Silktran, Synlube MT, Eneos MT, RoyalPurple Maxgear, Torco RTF, Neo 75w90HD, Idemitsu MTF(?)..........
 
Originally Posted By: LoneRanger
According to the owner's manual as well as the 2009 Forester Factory Service Manual, yes. Both of them specify the same gear oil type for rear diff and for the manual trans: API GL-5, @ 75W90 'recommended.' They have the usual ambient temperature chart with 75W90 covering the broadest range of course.

Then here would be my short list in no particular order:

- Motul Gear 300
- RLI BioSyn 75w-90 (not on the website)
- Amsoil SVG
- Red Line 75w90NS
 
For the transaxle, you want a GL5 that is specifically blended for MANUAL TRANS use. The front trans final drive is not a hypoid gear so the need for an oil that is specifically blended for them. For the rear, use a GL5 is is specifically blended for differentials.
 
Correction: the center diff is a sealed unit with it's own lube. It does *not* share the MT tranny fluid. However, the front diff does.

I use Motul Gear 300 in the tranny and rear diff in my 08 Forester. I currently have 25k miles on the current fill and it still shifts like a dream.

I do recommend changing out the rear diff after about 5-10k miles. The OEM fluid is a mess after break in since rear diff sheds metal like crazy in the first few thousand miles.
 
I really like the amsoil svg in my subaru. I've also used redline 75w90ns with good results, but the amsoil does shift slightly better for me.
 
Yo Ranger, welcome to thew world of Subaru. Here is the dilemma. The manual tranny shares the same lube as the front differential. Not a real good idea, but one we have to live with and make the best of.

The front hypoid differential wants a GL5 gear oil to keep it happy since the slip-sliding of the pinion on the ring gear requires a lot of extreme pressure (EP) additives.

The tranny would be happier with a GL4 oil that is optimized for good synchro engagement and operation, and does not need all the EP additives that the hypoid needs. The EP additives are sulphur based and are corrosive to the yellow metals used on synchros.

In this shared-lube situation, the hypoid wins the arguement. You are required to use GL5 oil, but be careful. You want a GL5 WITHOUT the friction modifiers for limited slip (LS). If you use an oil with the LS friction modifiers, it will be too slippery and the synchros will not be happy and you will get grinding when you shift.

Here's the problem, most of the top grade synthetic GL5 oils have LS adds. Which ones don't?

Redline 75W90-NS
Motul Gear 300 75W90
NEO 75W90
Subaru Extra-S

I would suggest you go to a Subaru specific forum to discuss this in more detail with guys who are in the same boat as you (and I).

Right now I have the Extra-S in my Outback and it works pretty good. The local WRX/STI racers swear by the NEO. The Eggspurts over on Subaruforester.org swear by a cocktail of Motul Gear 300 and Redline Lightweight Shockproof.

Now, in the rear diff, you can use any good synthetic GL5 gear oil. It can have LS adds, doesn't matter.

Next time I do my front diff/tranny, I'm thinking about a cocktail of one quart Lightweight shockproof, one quart of Gear 300, and the rest NEO.

Oh, the proper size tool to remove the drain plug is a torx t-70.
 
Redline 75-90 NS would have been a a very good choice.
It can handle hypoid gearsets, yet is friction modified somewhat for good shifting.
 
Originally Posted By: mechtech2
Redline 75-90 NS would have been a a very good choice.
It can handle hypoid gearsets, yet is friction modified somewhat for good shifting.


Actually, the eggspurts on the Subaru forums do not recommend the Redline NS very much. But they do seem to be quite fond of the RL Lightweight Shockproof.

Oh, please do recall that friction modifiers which make it slipprier actually work against the synchros. Do you own a Subaru?
 
The Redline 75-090 NS is unusual in that it is one of the best hypoid gear lubes that is reasonable for shifting.
That is it's claim to fame.
 
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